P01 LACTATE TRAFFICKING INHIBITION RESTORES SENSITIVITY TO PROTEASOME INHIBITHORS AND ORCHESTRATES IMMUNOMICROENVIRONMENT IN MULTIPLE MYELOMA
Authors: C. Giallongo, T. Zuppelli, G. Scandura, E. La Spina, I. Dulcamare, D. Cambria, S. Giallongo, A. Romano, N. Parrinello, V. Del Fabro, M. Aguennoz, G. Li Volti, G.A. Palumbo, F. Di Raimondo, D. Tibullo
Journal: HemaSphere (2023)

Even though the advantages in therapeutic strategies, multiple myeloma (MM) remains an incurable disease as patients eventually develop relapsed or refractory MM. Metabolic changes of malignant plasma cells (PCs) and adaptation to tumor microenvironment represent determinant players in tumor cell growth, adaptation and resistance to anti-cancer therapy. Metabolic features of tumor PCs are typical of the Warburg effect with increased lactate production, but at the same time, MM cells can use the reverse Warburg effect taking up lactate from tumor microenvironment by using monocarboxylate transporter 1 (MCT1). We previously showed that MM mesenchymal stromal cells are more glycolytic and produce more lactate than healthy counterpart. Hence, we aimed to explore the effect of lactate on PCs and tumor microenvironment. Lactate concentration resulted increased in MM patient’s sera compared to healthy donors. Exposing MM cells to high levels of lactate resulted in a significant increase of MCT1, MCT4 and the lactate receptor GPR81. Interestingly, we also found a significant reduction of the apoptotic effect of proteosome inhibitors (PIs). Given that the uptake of lactate by myeloma PCs decreased anti-myeloma efficacy of PIs, we next investigated whether MCT1 is differentially expressed in primary CD138+ cells from MM at diagnosis and resistant/refractory patients. Analyzing BM biopsy specimens by using immunofluorescence, we found a higher colocalization of MCT1 within CD138+ PCs in resistant/refractory MM compared to patients at diagnosis. To elucidate whether the protective effect of lactate was mediated by metabolic activity of lactate or through the activation of GPR81 signaling, PCs were cultured in medium supplemented with 20% sera from different MM patients and treated with PIs alone or in combination with AZD3965, a selective inhibitor of MCT1, or 3-OBA, an antagonist of GPR81. Only combination with AZD3965 increased PI-induced apoptosis. These findings led us to examine metabolic effects of lactate in PCs. A significant increase of oxidative phosphorylation-related genes and mitochondrial ROS were observed after exposure to high lactate concentration. The lactate-induced oxidative metabolism was then confirmed by the higher basal respiration and mitochondrial ATP production found in PCs after exposure to lactate. However, growing MM cells for 3 days in medium supplemented with lactate resulted in a gradual decrease in cell growth. Also the oxygen consumption (OCR) and extracellular acidification rate (ECAR) were reduced, but the OCR/ECAR ratio was increased in respect of control, suggesting a higher contribution of mitochondrial respiration versus glycolysis to energy generation. This metabolic switch, resulting from chronic exposure to lactate, is essential to limiting the anti-myeloma effect of PIs. Finally, we explored the effects of lactate in promoting immune escape mechanisms in myeloma microenvironment. Lactate exposure increased of the amount of myeloid derived suppressor cells and Treg percentages. A significant reduction of both subtypes was found after blocking MCT1 by AZD3965. These data were confirmed using MM sera and blocking circulating lactate by AZD3965. Overall, these findings showed that targeting lactate trafficking in TME inhibits metabolic rewiring of tumor PCs and lactate-dependent immune evasion and thus improving therapy efficacy.